Large amounts of petroleum-based plastics are discarded every year, and therefore serious problems caused by such large amounts of waste, such as shortage of landfills and environment pollution, have been reported. Under the circumstances, biodegradable plastics have received attention which are decomposed by the action of microorganisms in the environment, landfills, or compost. Biodegradable plastics are under development with the aim of expanding their application to materials for use in the environment in agriculture, forestry, and fisheries, and also to food containers, package materials, sanitary materials, and garbage bags, which are difficult to recover/recycle after use.
Among them, from the viewpoint of reducing carbon dioxide emissions or fixing carbon dioxide (carbon neutral), polyhydroxyalkanoate (hereinafter, sometimes abbreviated as “PHA”) that is a plant-derived aliphatic polyester is receiving attention.
However, such polyhydroxyalkanoate has a slow crystallization speed, and therefore its molding processing requires a long cooling time for solidification after heat-melting, which causes problems such as poor productivity and change in mechanical properties with passage of time (especially, toughness such as tensile elongation at break) due to secondary crystallization that occurs after molding.
In order to solve such problems, addition of an inorganic substance such as boron nitride, titanium oxide, talc, lamellar silicate, calcium carbonate, sodium chloride, or metal phosphate to polyhydroxyalkanoate has heretofore been proposed to promote crystallization. However, addition of such an inorganic substance has many negative effects such as reduction in the tensile elongation of a resulting molded article, poor appearance of the surface of the molded article, and poor transparency of a resulting film, and therefore its effect is inadequate.
Other methods have been proposed to promote the crystallization of polyhydroxyalkanoate without using an inorganic substance. Examples of such methods include: one in which a natural product-derived sugar alcohol compound such as erythritol, galactitol, mannitol, or arabitol is added (Patent Document 1); one in which polyvinyl alcohol, chitin, or chitosan is added (Patent Document 2); one in which polyethylene oxide is added (Patent Document 3); one in which an aliphatic carboxylic amide, an aliphatic carboxylate, an aliphatic alcohol, or an aliphatic carboxylic acid ester is added (Patent Documents 4, 5, and 13); one in which a dicarboxylic acid derivative such as dimethyl adipate, dibutyl adipate, diisodecyl adipate, or dibutyl sebacate is added (Patent Document 6); one in which a cyclic compound having C═O and a functional group selected from NH, S, and O in its molecule, such as indigo, quinacridone, or quinacridone magenta, is added (Patent Document 7); one in which a sorbitol-based derivative such as bis-benzylidene sorbitol or bis(p-methylbenzylidene)sorbitol is added (Patent Document 8); one in which a cyclic compound having C═O and an NH group in its molecule is added (Patent Document 9); one in which a compound containing a nitrogen-containing heteroaromatic nucleus, such as pyridine, triazine, or imidazole, is added (Patent Document 10); one in which a phosphate compound is added (Patent Document 11); one in which a bisamide of higher fatty acid and a metal salt of higher fatty acid are added (Patent Document 12); and one in which branched polylactic acid is added (Patent Document 14). However, the fact is that a practically effective crystal nucleating agent has not yet been found.